Operons and gene regulation in bacteria

Question 1

Operons

Answer 1

Genes in an operon are transcribed as a group and have a single promoter.

Question 2

Regulatory DNA sequences

Answer 2

Something each operon contains that act as binding sites for regulatory proteins that promote or inhibit transcription

Question 3

Inducible operons

Answer 3

Operons that can be turned on by the presence of a particular small molecule

Question 4

Repressible operons

Answer 4

Operons on by default but can be turned off by a small molecule

Question 5

gene regulation

Answer 5

mechanisms for controlling which genes get expressed and at what levels

Question 6

What is found in the operons of bacteria

Answer 6

Related genes are often found in a cluster on the chromosome, where they are transcribed from one promoter as a single unit

Question 7

Cluster of genes under the control of a single promoter

Answer 7

Operons

Question 8

Regulatory proteins

Answer 8

Attach to regulatory DNA sequences that control how much operon is transcribed

Question 9

Repressors

Answer 9

Regulatory proteins that bind to pieces of DNA called operators. When bound to the operator it reduces transcription by blocking RNA polymerase from moving forward on the DNA

Question 10

Activators

Answer 10

Regulatory protein increases the transcription of the operon (by helping RNA polymerase bind to the promoter) when bound to its DNA site

Question 11

Structure of lac operon

Answer 11

Three genes lacZ, lacY, and lacA which are transcribed as a single mRNA under one promoter

lac operon also contains a number of regulatory sequences which are regions of DNA in which particular regulatory proteins can bind, controlling transcription of the operon

These 3 sites are the promoter, operator and CAP binding site

Question 12

Promoter

Answer 12

Binding site for RNA polymerase, the enzyme that performs transcription

Question 13

Operator

Answer 13

Negative regulatory site bound by the lac repressor protein. The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription

Question 14

CAP binding site

Answer 14

Positive regulatory site bound by catabolite activator protein. When CAP is bound to the site, it promotes transcription by helping RNA polymerase bind to the promoter

Question 15

Lac repressor

Answer 15

protein that represses (inhibits) transcription of the lac operon. It does this by binding to the operator, which partially overlaps with the promoter.

Question 16

What happens to the lac repressor when lactose is and isn’t present

Answer 16

When it is unavailable: The lac repressor binds tightly to the operator, preventing transcription by RNA polymerase

When it is available: The lac repressor loses its ability to bind DNA

Question 17

allolactose

Answer 17

Isomer (rearranged version) of lactose that causes the change in lac repressor.

When lactose is available it is converted to allolactose which binds to lac repressor and makes it change shape so it could not bind DNA

It is an example of an inducer, small molecule that triggers expression of a gene or operon

Question 18

lac operon

Answer 18

condiered an inducible operon because it is usually turned off (repressed) but can be turned on in the presence of inducer allolactose

Question 19

Catabolite activator protein (CAP)

Answer 19

CAP binds to region of DNA just before the lac operon and helps RNA polymerase attach to the promoter, driving high levels of transcription

Question 20

cyclic AMP

Answer 20

Regulates CAP since it is not always active

It is a hunger signal made by E.coli when glucose levels are low. cAMP binds to the CAP, changing its shape and making it able to bind DNA and promote transcription

Without cAMP CAP cannot bind DNA and is inactive

Question 21

Two conditions for lac operon to be expressed at high levels

Answer 21

Glucose must be unavailable: Since cAMP binds to CAp, making CAP bind to DNA. Bound CAp helps RNA polymerase attach to the lac operon promoter

Lactose must be available” If lactose is available the lac repressor will be released from the operator (by binding of allactose) Allows RNA polymerase to move forward on the DNA and transcribe the operon

Question 22

trp operon

Answer 22

group of five genes that encode biosynthetic enzymes for the amino acid tryptophan

Question 23

tryptophan

Answer 23

Amino acid needed by bacteria to build proteins

Question 24

trp operon structure

Answer 24

five genes that encode enzymes for trytophan biosynthesis
Promoter (RNA polymerase binding site)
Operator (binding site for repressor protein)

Question 25

trp repressor

Answer 25

regulatory protein that binds to the DNA of the operator. As a result it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme.

Question 26

How the presence of tryptophan effect transcription

Answer 26

trp repressor only binds and blocks transcription when tryptophan is present. This is because the tryptophan attaches to the repressor molecules and changes their shape so it becomes active

When there is little tryptophan in the cell, the trp repressor is inactive

Question 27

corepressor

Answer 27

small molecule that switches a repressor to its active state